The project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord which ultimately controls the activity patterns of motor units (motoneurons and the muscle fibers they innervate). Topics of interest include analysis of mechanisms of synaptic transmission in the spinal cord, of the reflex pathways within the spinal segment and control of information flow in them by input from primary afferent and supraspinal descending systems, and studies of the anatomy of spinal elements relevant to these physiological questions. Of particular current interest is the organization of synaptic input systems, both segmental and supraspinal, that project to particular motor pools and the interaction of these systems with the spinal mechanisms that generate rhythmic motoneuron output patterns underlying locomotion. Current work concerns the organization of excitatory last-order interneurons in the cat spinal cord, with particular reference to interneurons that transmit short-latency excitation from low-threshold skin afferents and from reticulospinal systems that travel in the medial longitudinal fasciculus (MLF). All of these interneuron groups are strongly influenced by the spinal central pattern generator (CPG) for locomotion. The differential patterns of CPG modulation indicate that separate systems of segmental interneurons, each with highly specific patterns of primary afferent and descending convergence, are present in the mammalian spinal cord. We have also studied the sources of variability of motoneuron excitability during monosynaptic reflexes, a subject with specific clinical applications.